This invention relates to homogeneous compositions containing a fluorinated solvent, hydrogen fluoride, and a co-solvent and the use of these compositions in cleaning and processing semiconductors and integrated circuits including silicon and Ga/As substrates.
The use of microelectronic devices, such as integrated circuits, flat panel displays and micro electromechanical systems, has burgeoned in new business and consumer electronic equipment, such as personal computers, cellular phones, electronic calendars, personal digital assistants, and medical electronics. Such devices have also become an integral part of more established consumer products such as televisions, stereo components and automobiles.
These devices in turn contain one or more very high quality semiconductor chips made from silicon wafers containing many layers of circuit patterns. Typically nearly 350 processing steps are required to convert a bare silicon wafer surface to a semiconductor chip of sufficient complexity and quality to be used, for example, in high performance logic devices found in today""s personal computers. The most common processing steps of semiconductor chip manufacture are wafer-cleaning steps, accounting for over 10% of the total processing steps. These cleaning steps are normally one of two types: oxidative and etch. During oxidative cleaning steps, oxidative compositions are used to oxidize the silicon or polysilicon surface, typically by contacting the wafer with aqueous peroxide or ozone solution. During etch cleaning steps, etching compositions are used to remove native and deposited silicon oxide films and organic contaminants from the silicon or polysilicon surface before gate oxidation or epitaxial deposition, typically by contacting the wafer with aqueous acid. See, for example, L. A. Zazzera and J. F. Moulder, J. Electrochem. Soc., 136, No. 2, 484 (1989). The ultimate performance of the resulting semiconductor chip will depend greatly on how well each cleaning step has been conducted.
Microelectromechanical systems (MEMS) (also called micromachines or micromechanical devices) are small mechanical devices that can be made using traditional integrated circuit manufacturing techniques. Typical devices include motors, gears, accelerometers, pressure sensors, actuators, mirrors, personal information carriers, biochips, micropumps and valves, flow sensor and implantable medical devices and systems. The manufacture of MEMS results in a chip, or die, which contains the moving pieces of the device made from silicon or polycrystalline silicon (polysilicon) encased in silicon oxide. The die can also contain the circuitry necessary to run the device. One of the final steps in the manufacture of MEMS is commonly referred to as release-etch and consists of an aqueous etch utilizing hydrofluoric acid (HF) to remove the silicon oxide to free, or release, the silicon or polysilicon pieces and allow them to move.
For etch cleaning steps, the composition of choice has been dilute aqueous hydrofluoric acid (HF) and, to a lesser extent, hydrochloric acid (HCl). Currently, many semiconductor fabricators employ an xe2x80x9cHF-lastxe2x80x9d etch cleaning process consisting of an etching step using dilute aqueous HF to etch oxides.
Another important cleaning process in semiconductor chip manufacture is the removal of residues left behind from plasma ashing or etching of dielectric, photoresist or metals. The removal of these xe2x80x9cpost-etch residuesxe2x80x9d is challenging because of their multicomponent nature (i.e., the residues are typically comprised of both organic and inorganic compounds) and because the residues are adjacent to sensitive device features that must not be damaged during residue removal. Etch cleaning processes directed at removing xe2x80x9cpost-etch residuesxe2x80x9d will often utilize an aqueous HF composition in a first step, followed by a multi-step process to remove inorganic components of the residue. For instance, ethylene glycol-HF-NH4F aqueous solutions are widely used for the removal of xe2x80x9cpost-etch residuesxe2x80x9d from metal lines, and dilute aqueous HF is often used to remove cap and side wall veil residues after shallow trench isolation etching. See, for example, S. Y. M. Chooi et al., Electrochem. Soc., Proceedings, xe2x80x9cSixth International Symposium on Cleaning Technology in Semiconductor Device Manufacturing,xe2x80x9d 99-35 (1999).
However, etch cleaning of silicon surfaces with aqueous HF compositions has presented many problems to the semiconductor chip manufacturer. For example, contact with aqueous HF compositions renders the silicon surface hydrophobic and thus very susceptible to contamination by particles such as silicon oxides and other inorganic and organic materials. To remove these particles, the etched wafer is typically rinsed with deionized water, ethyl alcohol or isopropyl alcohol and is dried prior to subsequent processing. Unfortunately, the rinse does not always effectively remove these residual particles from the wafer, as the low energy silicon wafer surface is not easily wet by aqueous or alcoholic rinsing compositions which inherently have high surface tensions. In addition, rinsing with deionized water gives rise to slow drying time, while rinsing with alcohol introduces a potential fire hazard.
Another problem with employing aqueous HF compositions for etch cleaning is the slow rate of etching realized, possibly caused by deactivation of HF by water. To overcome this slow etch rate, most aqueous HF etching compositions need to incorporate at least 0.5% HF by weight. The slow etch rate of aqueous HF solutions can be of particular importance for MEMS devices. Silicon oxide dimensions in MEMS vary but are typically on the order of 1 xcexcm thick with lateral dimensions of 10-500 xcexcm. Slower etch rates lead to longer processing times. Etch assist holes are often added to polysilicon structures for which large, thin regions of silicon oxide must be removed, such as for the release of micro-mirrors, in order to accommodate the slow etch rate of aqueous HF solutions and reduce etch times. The etch assist holes may adversely affect the ultimate device performance.
The compositions of the present invention may be used to prepare MEMS devices having a large critical etch dimension. The critical etch dimension is that distance that the etchant must travel to dissolve all the polysilicate glass and release the device from the silicon wafer. The present compositions can be used to release devices having a critical etch distance of 400 micrometers or more, and are preferably used to etch and release devices having a critical etch distance of 40 to 400 micrometers.
In one aspect, this invention relates to a cleaning composition useful in semiconductor and integrated circuit manufacture, the composition comprising a fluorinated solvent, hydrogen fluoride or onium complex thereof, and sufficient amount of a co-solvent to form a homogeneous mixture. Advantageously, the present invention provides a liquid substrate cleaning composition useful for etching, removal of residues, rinsing and drying that contains a relatively low concentration of HF, but has an efficient rate of etching. The present composition may also be rendered non-flammable by appropriate selection of the fluorinated solvent. Substrates useful in the present invention include silicon, germanium, GaAs, InP and other III-V and II-VI compound semiconductors. It will be understood, due to the large number of processing steps involved in integrated circuit manufacture, that the substrate may include layers of silicon, polysilicon, metals and oxides thereof, resists, masks and dielectrics.
The present invention is also particularly useful in the etch and release of microelectromechanical devices. The etch cleaning and drying of MEMS has similar issues to those for semiconductor chip manufacture. Particulate contamination on micromachines can hinder movement of the device and ultimately affect device performance or cause failure. Care is taken to rinse the device with deionized water followed by ethyl alcohol or isopropanol, particles are not easily removed from microelectromechanical devices due to the polysilicon surface energy and intricate designs.
In addition to the problem of particulate contamination, drying of MEMS following deionized water rinses or alcohol rinses can lead to a phenomenon known as stiction. Stiction can be described as the adhesion of two surfaces due to adhesives forces as well as frictional forces. Polysilicon devices are typically 0.2-4.0 xcexcm, but can range up to hundreds of xcexcm, with lateral dimensions anywhere from 1-500 xcexcm. The high surface area of these structures along with the tight tolerances between structures makes stiction a very troublesome problem. Stiction of microdevices can occur during use of the device or as a result of capillary effects during the drying of the device following the release etch process. See, for example, R. Maboudian and R. T. Howe, J. Vac. Sci. Technol. B, 15(1), 1-20 (1997). The high surface tensions of the aqueous or alcoholic rinses can greatly exacerbate the capillary effects and lead to a higher incidence of microstructure stiction following the release-etch and drying steps.
In yet another aspect, this invention relates to a cleaning process for silicon or polysilicon part in MEMS chip with a homogeneous cleaning composition comprising a fluorinated solvent, HF and co-solvent. The present invention provides a wafer cleaning composition with low surface tension that easily penetrates the intricate microstructures and wets the surfaces on MEMS substrates. The cleaning composition is easily removed from MEMS and provides a dry, hydrophobic surface without residual or trapped water that could be present from a high surface tension aqueous cleaning composition. In contrast to the prior art, the present invention provides a method for the etch and release of microelectromechanical devices that etches and releases MEMS with no, or fewer, etch assist holes in MEMs device. Additionally the composition etches and releases while preventing or reducing stiction between said MEMs elements.
As used herein, xe2x80x9cmicromechanical devicexe2x80x9d refers to micrometer-sized mechanical, optomechanical, electromechanical, or optoelectromechanical device. Various technology for fabricating micromechanical devices is available using the Multi-User MEMS Processes (MUMPs) from Cronos Integrated Microsystems located at Research Triangle Park, North Carolina. One description of the assembly procedure is described in xe2x80x9cMUMs Design Handbookxe2x80x9d, revision 5.0 (2000) available from Cronos Integrated Microsystems.
Polysilicon surface micromachining adapts planar fabrication process steps known to the integrated circuit (IC) industry to manufacture microelectromechanical or micromechanical devices. The standard building-block processes for polysilicon surface micromachining are deposition and photolithographic patterning of alternate layers of low-stress polycrystalline silicon (also referred to as polysilicon) and a sacrificial material (e.g., silicon dioxide or a silicate glass). Vias etched through the sacrificial layers at predetermined locations provide anchor points to a substrate and mechanical and electrical interconnections between the polysilicon layers. Functional elements of the device are built up layer by layer using a series of deposition and patterning process steps. After the device structure is completed, it can be released for movement by removing the sacrificial material using a selective etchant such as hydrofluoric acid (HF) which does not substantially attack the polysilicon layers.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane, and additional layers of mechanical polysilicon which can be used to form functional elements ranging from simple cantilevered beams to complex electromechanical systems. The entire structure is located in-plane with the substrate. As used herein, the term xe2x80x9cin-planexe2x80x9d refers to a configuration generally parallel to the surface of the substrate and the terms xe2x80x9cout-of-planexe2x80x9d refer to a configuration greater than zero degrees to about ninety degrees relative to the surface of the substrate.
Typical in-plane lateral dimensions of the functional elements can range from one micrometer to several hundred micrometers, while the layer thicknesses are typically about 1-2 micrometers. Because the entire process is based on standard IC fabrication technology, a large number of fully assembled devices can be batch-fabricated on a silicon substrate without any need for piece-part assembly.
The present composition may also make advantageous use of hydrogen fluoride onium complexes, discussed in greater detail below, which are safer and more easily handled that anhydrous hydrogen fluoride. Thus in the present invention, hydrogen fluoride will be used to denote both anhydrous hydrogen fluoride as well as the onium complexes of hydrogen fluoride.
In yet another aspect, this invention relates to a cleaning process for substrates comprising contacting a substrate with a homogeneous cleaning composition comprising a fluorinated solvent, hydrogen fluoride (or onium complex thereof); and sufficient amount of a co-solvent to form a homogeneous mixture; and separating the cleaning composition from the processed substrate. The cleaning process makes more efficient use of the available HF than conventional aqueous processes and achieves an etch cleaning rate comparable to that of conventional aqueous HF compositions but with a relatively low HF concentration. The lower HF concentrations thus improve product safety while reducing adverse environmental impact.
In yet another aspect, this invention relates to a method for terminating the etching process performed by a hydrogen fluoride-containing etching composition, the method comprising the steps of: (1) providing a substrate; (2) contacting the substrate with a cleaning composition comprising hydrogen fluoride, a fluorinated solvent; and sufficient amount of a co-solvent to form a homogeneous mixture; (3) allowing sufficient time for the composition to etch the substrate to the desired extent; and (4) adding a sufficient amount of an alcohol to the etching composition to terminate the etching process.